1. Field of the Invention
This invention relates to a distance measuring method and apparatus suitable for use for measurement of a shape of a three-dimensional object.
2. Description of the Related Art
In distance measurement based on an optical cutting method which makes use of slit light, a three-dimensional position of a subject for measurement can be determined based on triangulation using, as shown in FIG. 13, a direction of projection of slit light from a light source, a sight line direction of a camera including a lens and an imaging plane and a positional relationship between the light source and the camera.
Recently, much effort is directed to investigations for scanning of slit light with a rotary mirror to effect such distance measurement as described above at a high speed, and various methods have been proposed. In a most popular one of the methods, a plurality of cells are disposed in a matrix to form an imaging plane, and a point in time at which slit light passes a point (small area) of a subject for measurement which is being measured by each cell, that is, a point in time when the slit light reflected from the subject for measurement passes each cell, is detected. Then, based on the points in time thus detected, an angular position of a mirror by which the slit light is being scanned is determined to make a distance measurement. The method is disclosed in detail, for example, in Japanese Patent Laid-Open Application No. Showa 62-228106.
In the conventional distance measuring method described above, one photo-sensor, such as, for example, a photodiode or a phototransistor, is disposed for each of the cells of the imaging plane, and a threshold level is set for photoelectric currents outputted by photo-sensors to detect passage of the slit light. Accordingly, the conventional distance measuring method is disadvantageous in the following points:
Firstly, the amount of slit light reflected from the subject for measurement and projected to the imaging plane and the spatial extent of the slit light in its widthwise direction, refer to FIG. 14, depend upon a condition, a direction and so forth of a surface of the subject for measurement. As shown in FIG. 15, when the amount of light is small, the photoelectric current of a photo-sensor may not exceed the threshold level, and consequently, the slit light cannot be detected. On the other hand, when the amount of the slit light differs, the variation in photoelectric current flowing through a cell with respect to time also differs, and some displacement is produced at a point of time when the slit light is detected with the fixed threshold level, resulting in an error in distance measurement.
Secondly, when the temperature of a photo-sensor on the imaging plane rises as a result of measurement for a long time or by some other cause, dark current varies, and consequently, passage of the slit light cannot be detected accurately. Accordingly, temperature compensation is required.
Thirdly, if received light is integrated and a resultant integration value is read out, then the sensitivity is improved so that it becomes possible to measure the distance to an object at a long distance or an object having a comparatively low reflection factor. In this instance, however, when bias light, which is light from any subject for measurement in a common environment in which the slit light is not received, from a point being measured by a certain cell is different from bias light from another point being measured by another cell positioned at a different location on the same imaging plane, the threshold level must be adjusted for each of the cells. However, it is difficult to adjust the threshold level for each cell. Accordingly, it cannot be avoided to employ such a countermeasure as to adjust the environment so that no difference in bias light may be produced between different measurement points or to make reflected light of the slit light extremely higher than bias light. This makes an obstacle to a measurement of common environment. This is the reason why a differentiation method which is low in sensitivity is adopted widely.